I think it depends on several factors that make it difficult to determine who would win a knockdown fight between Mongul and Thanos. I think the deciding factor is: Is Thanos wielding the Infinity Gauntlet? If so, then the fight is over...quickly with Thanos standing over Mongul's corpse or whatever Thanos imagines. However, let's assume that Thanos does not have the gauntlet. That would make the fight much more even. Then it would be a toss up in my opinion given that the two are so powerful and ruthless. I'd put my money on Thanos to win because I think he'd be willing to do anything to impress Death and handing Mongul over to her would make her happy wouldn't it?